docs: def _open(filename, mode='r', buffering=0):
      """read only version of open()"""
      if mode not in ('r', 'rb', 'rU'):
          raise RuntimeError("Invalid open file mode, must be 'r', 'rb', or 'rU'")
      if buffering > MAX_OPEN_BUFFER:
          raise RuntimeError("Invalid buffering value, max buffer size is {}".format(MAX_OPEN_BUFFER))
      return open(filename, mode, buffering)